1. Field of the Invention
The present invention relates to a technique for splicing or coupling optical fibers to one another, particularly optical fibers that contain cores or other structures that are twisted about the axes of the fibers.
2. Discussion of the Known Art
Before coupling two optical fibers to one another such as by fusion splicing, the end faces of the fibers must be cleaved and aligned to confront one another so that the cores of the fibers can be fused to one another with negligible attenuation of signals that will pass between the fused cores. Most popular fusion splicers use a so-called profile alignment system (PAS) to effectuate the fiber alignment and fusing operations. In a PAS, two cameras are positioned to produce two side view images of a confronting end region of each fiber, such that the views are taken 90 degrees apart in a plane normal to the fiber axis. The cameras also serve to measure and obtain a profile of the brightness of each image from one side of the fiber to the diametrically opposite side, at a small offset distance axially from the end face of the fiber. The offset distance serves to avoid optical noise and issues relating to the quality of the cleaved end faces.
Fusion splicers that operate by directly imaging the end face of each fiber in the axial direction are also known, and thus may be capable of handling fibers containing one or more cores. For example, a CAS-4000 series of fusion splicers offered by Vytron of Morganville, N.J. Such fusion splicers are more complicated and at least double the cost compared to PAS splicers, however.
As mentioned, PAS fusion splicers display two side views of the end region of each fiber, one looking in an “X” direction in a plane normal to the length of the fiber and the other in a “Y” direction, wherein the X and the Y views are separated by 90 degrees in the normal plane. The relative alignment of the two fiber ends is adjusted either automatically by the fusion splicer, or manually until the X and the Y brightness profiles of the fibers are substantially identical to one another, or, if the fibers are dissimilar, until the profiles show a certain desired alignment. The end faces of the fibers are advanced (Z-direction) to abut one another, and the fibers including their cores are fused together by an electric arc discharge. See, U.S. Pat. No. 5,638,476 (Jun. 10, 1997); U.S. Pat. No. 6,791,067 (Sep. 14, 2004); and U.S. Pat. No. 7,003,200 (Feb. 21, 2006), all of which are incorporated by reference.
For example, a model S183PM II fusion splicer available through OFS Fitel, LLC, uses an active four-dimensional alignment system (X, Y, Z, and θ-rotation about fiber axis) to align various kinds of optical fibers to be spliced including polarization maintaining (PM) fibers, and a controllable electric arc to melt the fiber glass and fuse the fibers together. The system can operate to splice either identical or dissimilar optical fibers based on the obtained brightness profiles and/or other optical characteristics of the fibers, and a strong physical joint between the fibers with very low power loss or back-reflection (mismatch) is achieved.
A proper alignment of two fibers to be spliced may also be obtained experimentally by connecting one of the fibers to a light source and the other fiber to a power meter, and selecting an alignment that maximizes power output after performing a series of alignment adjustments. Apart from consuming much time, such a solution may also require stripping of light in the cladding modes of certain fibers, however.
Fibers containing multiple cores are now in use. See, e.g., U.S. Pat. No. 5,706,380 (Jan. 6, 1998), and U.S. Pat. No. 6,711,333 (Mar. 23, 2004). Methods of splicing multiple core fibers are disclosed in the mentioned '380 patent, and in U.S. Pat. No. 6,148,639 (Nov. 21, 2000). Also, a PAS fusion splicer previously offered by Ericsson as model FSU-995PM had a program (#60) that was listed as multicore.
The cores in some multiple core fibers may also be twisted helically about the axis of the fiber at a specified rate. Such fibers are used, inter alia, for sensing applications in the medical field wherein gratings are written in the fiber, and a fanout is provided to access each of the cores in the fiber.
If two twisted multiple core fibers are to be fusion spliced, it may be difficult to use existing PAS type splicers because, as mentioned, the brightness profiles are measured at positions that are offset axially from the end faces of the fibers. Therefore, even if a conventional PAS splicer determines, based on the measured profiles, that the fibers and their cores are aligned properly to be fused, the cores may nevertheless be misaligned at the fiber end faces because of the additional core twist from the positions of the profiles to the end faces. If the fibers are fusion spliced in such a condition, then signals passing between the fused cores will be attenuated at an unacceptable level.
Accordingly, there is a need for a fusion splicing technique that is capable of splicing fibers containing multiple cores, particularly twisted multiple cores, in a way that achieves a strong physical joint between the fibers and introduces only a negligible degree of attenuation between the spliced cores.